The present invention relates to a method of cleaning a filter unit of the type comprising a plurality of filter elements extending from a plate dividing the unit into a lower inlet side for gas carrying product and an upper outlet side for filtered gas. During cleaning-in-place of the filter elements in between periods of operation of the filter unit a cleaning liquid and pulses of pressurized gas are mixed on the outlet side of the filter element and supplied to the interior of the filter element.
Such a method is disclosed in WO 97/14288 which describes a spray drying apparatus with an integrated filter unit. During operation of the apparatus pressurized gas is supplied from the outlet side of the filter unit by means of nozzles placed immediately above the openings of the filter elements in reverse flow to the interior of the filter elements in order to prevent excessive product build-up on the external filter surface. In addition the filter elements are cleaned between periods of operation by supplying a cleaning liquid through nozzles to the interior of the filter elements. To promote the distribution of the cleaning liquid over the entire internal surface of the filter walls the cleaning liquid is mixed with pulses of pressurized gas. The cleaning of the filter elements from the inside may be assisted by a simultaneous washing of the external side of the filter elements.
Danish Patent Application DK 795/93 describes a method of cleaning porous filter elements of the type used for removing particles from a process gas. The filter unit may be external to or integrated in a drying, granulating, coating or spray drying device. Between periods of operation, cleaning liquid is supplied to and distributed over the internal filter surface by means of circular channels arranged at the outlet side of the filter unit around the openings of the filter elements. At the same time pulses of pressurized gas, such as air, are supplied through nozzles to the interior of the filter elements in order to support the cleaning effect. The cleaning liquid can also be supplied by means of nozzles. During operation the filter unit is cleaned by supplying reverse flow pulses of pressurized gas to the filter elements.
U.S. Pat. No. 4,731,100 describes a method of cleaning a gas filtering apparatus between periods of operation. A plurality of filter elements are vertically suspended in a plate and they can be washed on the product side by several radially displaced spray nozzles directing cleaning liquid to the external surface of the filter elements, and on the outlet side for filtered gas by spray nozzles placed inside the filter elements. When washing is effected the spray nozzles are displaced vertically in order to wash the suture filter surface. The cleaning effect can be intensified by switching on a conventional pneumatic cleaning system during the washing of the filter elements. The displaceable nozzles make the system complicated and time consuming to operate.
From FR-A 2702672 it is known to have a filter unit which is divided by a plate carrying filter elements and has liquid nozzles positioned in the plate between the elements in order to spray water out horizontally onto the surface wall of the elements.
The known methods of cleaning filters when not in operation suffer from several disadvantages, the major one being a requirement for large flow rates of cleaning liquid in order to obtain filters cleaned to a satisfactory degree before they once again are set in operation. The high flow rates have been obtained by using excessively large quantities of liquid or by recycling collected cleaning liquid with the consequence that small product particles dispersed in the cleaning liquid are transferred to the clean outlet side. The flow rate of cleaning liquid in the prior art filters is typically larger than 500 liters per hour per square meter of filter wall surface. The total amount of liquid spent in one cleaning-in-place operation with prior art systems using recirculation is typically in the order of 30 to 50 liters per square meter of filter wall surface for each cleaning step and, typically, at least 3 to 6 cleaning steps are required in these prior art methods using recycling.